A plurality of dimples are arranged on the surface of a golf ball for the purpose of reducing the air resistance of the ball in flight. From the standpoint of further improving the aerodynamic symmetry so that the ball may exert consistent flight performance independent of the point of impact, it is desirable to arrange the dimples on the golf ball surface as uniformly as possible.
Known approaches for the uniform arrangement of dimples on the golf ball surface include the use of spherical polyhedral arrangement patterns such as spherical icosahedral, spherical dodecahedral and spherical octahedral arrangement patterns as described, for example, in JP-A 2000-70413. For instance, the spherical icosahedral arrangement pattern is derived by assuming the golf ball surface to be a spherical icosahedron defining twenty triangular units, arranging dimples appropriately within each triangular unit in a good balance, and expanding them over the entire spherical surface.
Generally, golf balls are manufactured by injection molding. The injection mold consists of a pair of mold halves mated along a parting line which is in alignment with one great circle, known as equator, of the golf ball being molded therein. If it is desired to lay some dimples across the equator of the golf ball, the mold must be provided with dimple-forming protrusions across the parting line. This complicates the fabrication of the mold. It is then a common practice to avoid the design of disposing dimples across the equator of the golf ball. However, if no dimples are formed across the equator of the golf ball, the golf ball has an endless land formed along its equator, which means that the spherical polyhedral arrangement is distorted or disordered at this position.
Aside from the above-discussed concept of spherical polyhedral arrangement, a sort of polyhedral arrangement is also known as shown in FIGS. 10 to 12. In this method, an equator and a plurality of reference longitudes extending between a pair of poles divide the spherical surface into spherical triangles, and dimples are arranged within each spherical triangle as a reference.
FIG. 10 is a plan view of a prior art golf ball 5 having dimples of the polyhedral arrangement, as viewed from above one pole. FIG. 11 is an elevational view of the ball as viewed from above the equator. In the golf ball 5, six reference longitudes 52, depicted by dashed lines, extend from one pole 51 to the other pole 51 and are equally spaced. These six reference longitudes 52 and the equator 53 divide the spherical surface into twelve spherical triangle regions. A number of dimples 54 are arranged within each spherical triangle region such that the dimples in two adjacent spherical triangle regions sharing one side are in axial symmetry with respect to that boundary line. The same applies to the opposing hemisphere delimited by the equator.
With such a dimple arrangement, those dimples in opposite equator-adjoining portions are juxtaposed side by side as best shown in FIG. 11. The arrangement of dimples which are juxtaposed in pairs in a strip-like area straddling the equator is often considered unfavorable to an esthetic appearance.
The esthetic appearance of dimple arrangement may be improved if the dimple arrangement center line which is in alignment with each reference longitude 52 which is used as a reference in arranging dimples in each triangle region is shifted a predetermined distance between opposite hemispheres after dimples were arranged. FIG. 12 is an elevational view of a prior art golf ball 6 having dimples of such modified polyhedral arrangement, as viewed from above the equator. In the golf ball 6, dimple arrangement center lines 62, 62 on opposite hemispheres are shifted, in a rotational direction about an axis passing a pair of poles 61, 61, by a predetermined distance 6d, expressed as a shift or distance along the equator 63. Then, those dimples in opposite equator-adjoining portions are juxtaposed alternately or in zigzag. As a result, the esthetic appearance of the golf ball is improved.
However, arranging dimples with reference longitudes shifted can invite a degradation of the point symmetry of dimple arrangement with respect to the center of the golf ball, that is, a degradation of the symmetry of dimple arrangement, leading to a lowering of flight performance. Additionally, such a dimple arrangement adds to the manufacturing cost of golf balls for the reason described below.
When a golf ball of multilayer construction is manufactured by injection molding, the mold is generally provided, at positions located near the north and south poles 61 and 61 and aligned with dimples, with a plurality of support pins for holding a golf ball inner layer, typically a core, in place within the spherical cavity. Since the use of the above-described dimple arrangement results in a degradation of the point symmetry of dimple arrangement with respect to the center of the golf ball as described above, the positions of support pins are not in register between upper and lower mold halves. This negates the share of common parts and needs an accordingly increased expense.
Where a seamless array of dimples at the position of equator 63 is employed, the parting planes of upper and lower mold halves must be alternately corrugated or raised for mutual engagement. This engagement configuration cannot be arrived at by simply shifting upper and lower mold halves.